Every 2 days, our primary instance hits 100% CPU usage and has to be restarted. By setting the profiling level to 1, I discovered that there are about a dozen EXTREMELY slow count() queries being completed per second:

> db.system.profile.find()
{ "ts" : ISODate("2012-12-16T20:31:39.078Z"), "op" : "command", "ns" : "stylesaint.$cmd", "command" : { "count" : "tears", "query" : { "_id" :

{ "$gt" : ObjectId("50cdeadeaf58d3de96000294") }

, "active" : true, "is_image_processed" : true, "hidden_from_feed" : false, "hidden_from_public_feeds" : false }, "fields" : null }, "ntoreturn" : 1, "responseLength" : 48, "millis" : 13930, "client" : "wouldntyouliketoknow" }
... other similar results, only the ObjectId is different

I've run explain() on a number of these queries. Mongo seems to intelligently choose the index that gives it the fewest number of scanned documents, which is cool. But it means that one explain() does not explain them all. Here's an example:

> db.tears.find({ "_id" :

{ "$gt" : ObjectId("50cdeadeaf58d3de96000294") }

, "active" : true, "is_image_processed" : true, "hidden_from_feed" : false, "hidden_from_public_feeds" : false }).explain()
{
"cursor" : "BtreeCursor id",
"isMultiKey" : false,
"n" : 4,
"nscannedObjects" : 5,
"nscanned" : 5,
"nscannedObjectsAllPlans" : 23,
"nscannedAllPlans" : 25,
"scanAndOrder" : false,
"indexOnly" : false,
"nYields" : 0,
"nChunkSkips" : 0,
"millis" : 0,
"indexBounds" :

{ "_id" : [ [ ObjectId("50cdeadeaf58d3de96000294"), ObjectId("ffffffffffffffffffffffff") ] ] }

,
"server" : "ip-10-169-65-66:27017"
}

I have not found any queries yet where nscanned is more than a few hundred documents.

This particular collection has less than half a million documents. Whether or not these slow queries are related to the server failure, I'd like to speed them up. But my main issue is the server failure.

I created a topic with the Google group. We can take the discussion there if you prefer: https://groups.google.com/forum/#!topic/mongodb-user/NbiWPSeYg28